The present invention relates to a magnetron used in microwave application apparatus, and particularly relates to a magnetron in which the intensity of vibratory noise, that is, sound pressure, due to electro-striction in the material of a feed-through capacitor which constitutes a filter circuit of the magnetron is suppressed.
Because of their high efficiency in generating high frequency output, magnetrons are applied widely in the fields of radars, medical appliances, microwave ovens, and other microwave application apparatus.
Such a magnetron of this type is provided with a body portion having a resonance cavity, high frequency output portion and so on, and a filter portion for suppressing leakage of electromagnetic waves in a power supply portion for supplying the body portion with electric power.
The filter circuit is constituted by two choke coils and a feed-through capacitor. The choke coils are connected in series, at their respective first ends, to two externally led-out leads for supporting a filament provided in the resonance cavity and for supplying the filament with electric power. The feed-through capacitor is connected in parallel to the respective second ends of the choke coils.
The feed-through capacitor is passed through the wall of a filter casing in which the filter circuit is received from the inside to the outside so as to be connected to an external power supply.
As literature disclosing a conventional technique relating to a feed-through capacitor of this type which uses a commercial power source, refer to Japanese Utility Model Publication No. 57-56504.
The feed-through capacitor constituting the filter circuit provided in the magnetron uses a high dielectric constant ceramic material as its material. An alternating high voltage of several kV.sub.p-p having a frequency in a range of from 20 to 1,000 Hz is applied across the opposite terminals of the feed-through capacitor in operation of the magnetron.
In the commercial power source driving method, therefore, the ceramic constituting the feed-through capacitor is vibrated by electrostriction so as to produce sound waves of high sound pressure. The sound waves are radiated as unpleasant scoustic noise.
In order to reduce production of such electro-striction, use has been considered of a material of small dielectric constant e as the material constituting the feed-through capacitor. In the case of using such a small dielectric constant, the electrostatic capacity becomes small. In order to secure sufficient electro-static capacity necessary for the filter circuit for suppressing leakage of electric waves, it is necessary to reduce the distance between the electrodes of the feed-through capacitor or to increase the electrode area.
If the inter-electrode distance is made small, however, the breakdown voltage characteristic is lowered, while if the electrode area is increased, the size of the feed-through capacitor per se becomes large preventing miniaturization of the magnetron assembly.